The Solar Aspect Monitor (SAM) on NASA’s Solar Dynamics Observatory
- Extreme ultraviolet Variability Experiment (SDO-EVE) is a pinhole
camera.It utilzes a metal foil
filter and a small, pinhole aperture, to image the Sun in soft X-rays (0.1
– 7 nm) on a corner of one of the EVE CCD detectors.Some astute observers have noticed that
sometimes there is a dark spot just off the limb of the Sun.This is due to a blemish on the CCD,
not an actual object in space.The
first image below (Fig. 1) is a flat field of the portion of the CCD where the
SAM images are located.The white
spots and streaks are transient charged particle hits during the flat
fielding.The dark spot is a
permanent feature of the CCD response, basically insensitive pixels.The instrument is aligned and pointed
such that the dark spot is off the disk of the Sun (see Fig. 2), so the spot
cannot be seen against the black of deep space.Sometimes, however, there is an appropriately placed active
region on the limb of the Sun with material in the upper atmosphere of the Sun
(the corona) that is hot and emitting X-rays that SAM can detect.If that off-limb signal falls in the
area of the CCD blemish, then the dark spot becomes readily visible (see Fig.
3).

Figure
1.A flat field (uniform
illumination) of the SAM portion of the EVE CCD detector.The blemish is shown as a dark
collection of pixels.

Figure
2.A typical SAM solar image with
the CCD blemish off the solar disk not visible against black space.